Recently, optical systems to image a projected material in a space using a reflective imaging element are proposed (for example, PTLs 1 through 3). The optical system has a reflective imaging element and a projected material, and the image displayed in a space (hereinafter, referred to as “aerial picture”) is an image of the projected material imaged in a plane symmetric position with the reflective imaging element as a plane of symmetry. Such optical system utilizes specular reflection of the reflective imaging element, and in principle, a ratio in size of the image of the projected material to the image appeared in the space is 1:1.
As the reflective imaging element, one provided with holes penetrating through a substrate in a flat plate shape in a thickness direction and having an optical element configured with two mirror elements (also referred to as “unit imaging elements”) perpendicular to an inner wall of each hole (for example, refer to FIG. 4 in PTL 1), or one having an optical element provided with a plurality of transparent tubular bodies projected in a direction of a thickness of a substrate and configured with two mirror elements perpendicular to an inner wall surface of each tubular body is disclosed (for example, refer to FIG. 7 in PTL 1).
The reflective imaging elements disclosed in PTLs 1 and 2 have a substrate with a thickness from 50 μm to 200 μm having from several tens of thousands to several hundreds of thousands of square holes of approximately 50 μm to 200 μm on a side formed therein and an inner surface of each hole is subjected to mirror coating by electrocasting, nanoprinting, or sputtering. In particular, PTL 2 discloses a reflective imaging element allowing observation of an aerial picture by many people from various directions.
A reflective imaging element disclosed in PTL 3 has a reflective imaging element with a continuously varying thickness to make a wedge shape, thereby widening a viewing angle of an aerial picture.
For reference, contents of disclosure in PTLs 1 and 2 are incorporated herein in its entirety.